1. Field of the Invention
The invention relates to a method of localizing an object in a turbid medium, which method includes the following steps:
irradiation of the turbid medium, PA1 measurement of intensities of a part of the light transported through the turbid medium along a plurality of light paths from a source point where the light enters the turbid medium to a measuring point where the light leaves the turbid medium, PA1 reconstruction of an image of the interior of the turbid medium from a combination of the measured intensities. PA1 a light source for irradiating the turbid medium, PA1 means for coupling the light to be generated from the light source into the turbid medium at different angles, PA1 a photodetector for converting the light transported through the turbid medium into a photodetector signal, PA1 means for converting the photodetector signal in measured intensities, and PA1 a control unit for reconstructing an image of the interior of the turbid medium from the measured intensities. PA1 V represents the volume of a volume element j in the turbid medium, PA1 s(i)-d(i) represents a distance between a light source in a position s(i) and a measuring position d(i) for a measurement (i), PA1 s(i)-po) represents a distance between the position of the light source s(i) during the measurement (i) and the position p(j) of a volume element (j) for which the variation of the attenuation coefficient .kappa. is determined, PA1 p(j)-d(i) represents the distance between the position of said volume element j and the measuring position d(i) during the measurement (i), and PA1 F(s(i),d(i),.kappa.) represents the relative variation of the measured intensity in a position d(i) of a light source in a position s(i). PA1 .mu.'.sub.s is the inverse transport-corrected free path length, and PA1 .mu..sub.a represents the absorption coefficient. PA1 determination of estimated intensities from a transformation of a predetermined first image, PA1 determination of a difference between the measured intensities and the estimated intensities, and PA1 determination of a next image from the first image, a convergence factor and a back-transformation of the difference, the convergence factor being chosen to be depending on the weighting factor. These steps are repeated for all light sources and measuring positions. The convergence factor can be chosen equal to a constant times the weighting factor. As a result of the introduction of a convergence factor equal to the weighting factor, in the algebraic reconstruction technique a change of the next image relative to the first image is influenced less by a measurement containing a large amount of noise than the change in the next image which is due to a measurement involving little noise. PA1 determination of variations of the attenuation coefficients of the light paths from the measured intensities, and PA1 determination of a variation of the attenuation coefficient of a volume element of the turbid medium by means of a back-transformation which contains a weighted mean value of an estimate of the inverse sensitivity matrix with the variations determined for the attenuation coefficients of the light paths, the effect of the noise factor on the reconstruction being determined by the product of the weighting factor and the elements of the estimate of the inverse sensitivity matrix. The transposed sensitivity matrix constitutes an example of an estimate of the inverse sensitivity matrix. The product of the weighting factor and the elements of the transposed sensitivity matrix makes it possible that the intensities measured with a high noise factor exert a small effect only on the reconstruction in comparison with measurements with a low noise factor. PA1 I.sub.measurement (m,n) represents the measured intensity of light having traveled along a light path in a turbid medium between a measuring light source m and a measuring position n, and I.sub.calibration (m,n) represents a previously measured intensity of light having traveled along a light path in the reference medium between the measuring light source m and the measuring position n.
The invention also relates to a device for localizing objects in turbid media, which device includes:
2. Description of Related Art
In the context of the present application the term light is to be understood to mean electromagnetic radiation of a wavelength in the range of from 400 to 1400 nm. Furthermore, a turbid medium is to be understood to mean a substance consisting of a material having a high light scattering coefficient. Examples in this respect are an Intralipid solution or biological tissue.
A method of this kind is known from the article "The forward and inverse problems in time resolved infra-red imaging", by S. R. Arridge, SPIE, IS11:35, 1993. The known method can be used for in vivo breast examinations. to determine the presence of tumors in breast tissue of a human or animal female. According to the known method the turbid medium is irradiated from different positions, intensities of light transported through the turbid medium being measured for one irradiation position in different positions where the light leaves the turbid medium. The known algebraic reconstruction technique constitutes an iterative method in which per iteration step a next image is determined in dependence on a sensitivity matrix and an intensity measurement and a previously determined image which, after the first iteration, is chosen to be equal to the image determined during the previous iteration step. Herein, a sensitivity matrix is to be understood to mean a matrix in which a matrix element (i, j) contains a sensitivity function, the row number (i) of the matrix element corresponding to a light source and a measuring position of an intensity measurement i whereas the column number (j) of the matrix element corresponds to a position in space of a volume element j. The sensitivity function of a matrix element A(i,j) is given by the formule: ##EQU1##
in which
Furthermore, an attenuation coefficient is to be understood to mean the inverse diffuse absorption distance .kappa., given by the formule EQU .kappa.=3.mu..sub.a +L .mu.'.sub.s +L ,
in which
It is a drawback of the known method that the image suffers from artefacts in the form of stripes which correspond to a path from a first position where light enters the turbid medium to a second position where light leaving the turbid medium has been measured. Furthermore, artefacts in the form of a more or less regular structure occur at the center of the image.
Citation of a reference herein, or throughout this specification, is not to construed as an admission the such reference is prior art to the Applicants' invention of the invention subsequently claimed.